Exposes default MessageHeaders for HttpRequests

[luizrsilva]

Hello,

I am implementing a SignalR Hub that will be managed by APIM Azure.
The company I work for requires a header for monitoring the application but the library doesn't expose any way to send different headers for the HttpRequest, only the accessTokenFactory.

I am openning this pull request to expose a MessageHeaders object so the client can send default headers.

Code Example

final defaultHeaders = MessageHeaders();
defaultHeaders.setHeaderValue("HEADER_MOCK_1", "HEADER_VALUE_1");
defaultHeaders.setHeaderValue("HEADER_MOCK_2", "HEADER_VALUE_2");

final httpConnectionOptions = new HttpConnectionOptions(
          httpClient: WebSupportingHttpClient(logger,
              httpClientCreateCallback: _httpClientCreateCallback),
          accessTokenFactory: () => Future.value('JWT_TOKEN'),
          logger: logger,
          logMessageContent: true,
          headers: defaultHeaders);

final _hubConnection = HubConnectionBuilder()
          .withUrl(_serverUrl, options: httpConnectionOptions)
          .withAutomaticReconnect(retryDelays: [2000, 5000, 10000, 20000, null])
          .configureLogging(logger)
          .build();

Http Request Log

I/flutter ( 5248): Starting connection with transfer format 'TransferFormat.Text'.
I/flutter ( 5248): Sending negotiation request: https://localhost:5000/negotiate?negotiateVersion=1
I/flutter ( 5248): HTTP send: url 'https://localhost:5000/negotiate?negotiateVersion=1', method: 'POST' content: '' content length = '0' 
headers: '{ content-type: text/plain;charset=UTF-8 }, { HEADER_MOCK_1: HEADER_VALUE_1 }, { X-Requested-With: FlutterHttpClient }, { HEADER_MOCK_2: HEADER_VALUE_2 }, { Authorization: Bearer JWT_TOKEN }'